import java.util.Scanner;

public class No143 {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner sc = new Scanner(System.in);
		int K = sc.nextInt();
		int N = sc.nextInt();
		int F = sc.nextInt();
		
		int sum=K*N;
		
		for(int i=0; i<F;i++)
			sum=sum-sc.nextInt();
		
		if(sum>=0)
			System.out.println(sum);
		else
			System.out.println(-1);
		
	}

}